Engle’s most important contribution was his path-breaking discovery of a method for analyzing unpredictable movements in financial market prices and interest rates. Accurate characterization and prediction of these volatile movements are essential for quantifying and effectively managing risk. For example, risk measurement plays a key role in pricing options and financial derivatives. Previous researchers had either assumed constant volatility or had used simple devices to approximate it. Engle developed new statistical models of volatility that captured the tendency of stock prices and other financial variables to move between high volatility and low volatility periods (“Autoregressive Conditional Heteroskedasticity: ARCH”). These statistical models have become essential tools of modern arbitrage pricing theory and practice.